Method for recovering useful products from waste products obtained when manufacturing aluminium fluoride

ABSTRACT

The invention relates to a method for recovering useable products from waste products deriving from the manufacture of aluminium fluoride on the basis of aluminium hydroxide and fluosilicic acid. Mother liquors and washing water from a scrubber in said process, which solutions contain aluminium, silica, fluorine, and phosphorous is reacted in a first step with sodium ions at pH 2-3, and a temperature of 50°-100° C. to give a precipitate of sodium fluoroaluminate, which is isolated. The filtrate from said first step can be reacted with the silica waste product obtained in the above mentioned aluminium fluoride manufacture.

DESCRIPTION

1. Technical Field

The present invention relates to a method for recovering useful productsfrom waste products obtained when producing aluminium fluoride fromaluminium hydroxide and fluosilicic acid; and more particularly torecovering sodium silicoaluminates and sodium fluoroaluminates fromsilica waste, mother liquor and wash-water deriving from the manufactureof aluminium fluoride.

An object of the present invention is to obtain a possibility ofrecovering useful and economically valuable products from the wasteproducts obtained in the aluminium fluoride production.

2. Background of the Invention

When manufacturing aluminium fluoride, several troublesome wasteproducts are obtained which must be deposited and, optionally, renderedharmless. Thus, there is obtained a silica waste which contains fluorineand aluminium impurities, and a mother liquor and a wash-watercontaining fluorine, aluminium and silicon. These waste products presenta serious problem, since they occur in relatively large quantities andrequire complicated and expensive treatment before they can bedeposited. It is true that silica residues per se can be deposited,but--as will be understood--the direct deposit of silica contaminatedwith large quantities of harmful impurities cannot be tolerated. Themother liquor and the wash water must be neutralized with calciumhydroxide to a pH-value of about 10, at which the fluorine and silicatecontents thereof precipitate. This precipitate is then separated fromthe mother liquor and wash water, for example by centrifugation, andsubsequently deposited.

Thus, the aforementioned waste products constitute a seriousenvironmental problem, even though they may be treated in the mannerindicated before being deposited. In addition, substantial amounts ofsilicon and fluorine are lost when the waste is treated in said manner.

The reactions involved in the production of aluminium fluoride are givenbelow, the waste products being underlined. ##STR1## Prior artrecognizes some different methods for recovering active silica andcryolite as well as other compounds from fluosilicic acid and wasteproducts from the preparation of aluminium fluoride.

Austrain,B, 314 474 discloses the production of aluminium fluoride fromfluosilicic acid recovered from the production of phosphate containingfertilizers. Hereby 8-25% fluosilicic acid is reacted with aluminiumhydroxide in a relation which is 0.95-1.05:1 of the stoichiometricrelationship according to the formula. H₂ SiF₆ +Al(OH)₃ →2AlF₃ +SiO₂ +H₂O at a temperature of 60°-100° C. The aluminium fluoride obtained can beused for the production of cryolite. Aluminium hydroxide is therebyreacted with fluosilicic acid in two steps, using one third of theamount of aluminium hydroxide requested in the first step, and twothirds thereof in the second step, whereby a mixture containingaluminium fluoride, which is precipitated and separated off, andaluminium hydrofluoric acid is obtained. The aluminium hydrofluoric acidis then treated with alkali at pH 3-6.5, preferably at pH 6-6.5 and at atemperature of 60° C. to form cryolite. This process, however, requiresvery pure components as otherwise contaminations of i.a. phosphates willprecipitate and contaminate the cryolite. Any silica present will alsoform complexes which contaminates the cryolite and deteriorates thequality thereof.

Using fluosilicic acid and ammonia as starting materials active silicaand cryolite can be prepared (Austrain, B, 315128). Thereby ammoniumfluoride and silica are primarily prepared, whereafter the ammoniumfluoride is reacted with sodium aluminate to the formation of cryolite.

Swedish,B, 366723 relates to a process to recover fluorine and aluminiumfrom dilute, aqueous solutions, particularly mother liquors obtained inthe crystallization of aluminium fluoride, whereby the mother liquorsare treated with ammonia, or ammonium dicarbonate to give achiolite-analogous ammonium compound, (NH₄)₅ Al₃ F₁₄. The precipitationof said compound is carried out at pH 6-6.5.

Swedish,B, 7502835-7 relates to a concentrated synthetic fluxing agentbased on calcium fluoride, where remaining fluorine quantities from theproduction of aluminium fluoride from fluosilicic acid and aluminiumhydroxide are precipitated using a calcium compound.

In accordance with Swedish,A, 8100218-0 a method for recovering usefulproducts from waste of the described form is disclosed, whereby silicaobtained in the manufacture of aluminium fluoride and contaminated with,inter alia, fluorine and aluminium, is dissolved in a strongly basichydroxide; mixing the first solution obtained with a second solutionobtained by dissolving aluminium hydroxide with a strongly basichydroxide, and with waste mother liquor and optionally also washingwater from the manufacture of aluminium fluoride in such proportionsthat the pH-value of the mixture lies between about 10 and 14, thesilica content of the waste products supplied being precipitated as asilicoaluminate, which is separated off, preferably by filtration, foroptional further treatment or for direct use, whereafter fluorine, ifpresent from the waste products, is recovered from the filtrate byadding thereto an aluminium compound in an amount sufficient toprecipitate substantially all the fluorine content contained in thefiltrate as a fluoroaluminate, which is separated off, preferably byfiltration, for further use, and whereafter the final filtrate is passedto a recipient or utilized, for example, as process water in otherprocesses.

Carrying out said method a sodium aluminium silicate is obtained whichis rather alkaline and will thereby require a high demand of alum in thepreparation of a paper from a paper pulp using said silicate as afiller.

DISCLOSURE OF THE PRESENT INVENTION

The present invention relates to a surprisingly distinctive method forrecovering valuable substances in the form of useful products from wasteof the aforedescribed kind, while at the same time substantiallyeliminating the environmental problems created by the waste. The methodaccording to the invention is of further economic interest, since, amongother things, the waste need not be neutralized, meaning that no calciumhydroxide need to be used, which in turn means lower costs.

By the present method it has been shown possible to prepare an aluminiumsilicate which is less alkaline than that described above as well as ithas been shown possible to precipitate a fluoroaluminate having a veryhigh purity and a crystal size that can be varied depending on simplevariations of the precipitation conditions.

The present method is thus characterized by adding alkali metal ions inthe form of an alkali metal salt to collected mother liquors and washingwater obtained in the manufacture of aluminium fluoride and beingcontaminated with aluminium, silica, and fluorine, and by adjusting thehydrogen ion concentration of said mother liquors and washing water to apH of 2.0 to 3.0 by adding an alkali metal base while maintaining atemperature of said collected solutions of above about 50° C., isolatingthe alkali metal fluoroaluminate thereby precipitated by filtration,whereafter the filtrate comprising silica and minor amounts of aluminiumand fluorine can be passed to a second step for the recovery of one ormore of said compounds, or otherwise used.

Further characteristics of the invention are set forth in theaccompanying claims.

The term alkali metal base above means any potassium, sodium, or lithiumhydroxide, carbonate, hydrogen carbonate or sulphite, preferably sodiumhydroxide and sodium carbonate.

Thus, the useful product produced is a fluoroaluminate, preferably asodium fluoroaluminate as chiolite or cryolite. In a second stage thesilica content of the waste products can be recovered in the form of asilicoaluminate, preferably sodium silicoaluminate. Such products assodium fluoroaluminate (cryolite), and sodium silicoaluminate can beused directly, and are requested by the market. Sodium fluoroaluminateor cryolite is used to a large extent as a fluxing agent when producingaluminium electrolytically. Sodium silicoaluminate can be used as apigment agent in the paper-making industry and in the rubber and paintindustry.

Cryolite is thus used mainly as a fluxing agent in the aluminiumindustry, but is also known as a fluxing agent when preparing enamels,as a filler in polishing/grinding discs, as an intensifying pigment, asa grinding powder, or as an insecticide.

According to the invention, the useable products are preferablyrecovered from the waste in sequential steps in a coherent process, inwhich first fluorine and aluminium are recovered in the form of afluoroaluminate, preferably sodium fluoroaluminate, and the siliconcontent in an optional subsequent step, is recovered in the form of asilicoaluminate, preferably sodium silicoaluminate.

When recovering the fluorine and aluminium contents of the wasteproducts, particularly the mother liquors and washing water (scrubberwater) from the aluminium fluoride production, for example in the formof sodium fluoroaluminate (cryolite), there are collected such motherliquors and/or washing water, the hydrogen ion concentration thereof iscontrolled to be about pH 1 whereupon the hydrogen ion concentration isadjusted by the addition of sodium hydroxide and/or sodium carbonate topH 2.0-3.0, more preferably 2.0-2.5. The temperature of the solution isbrought to or maintained at, if already hot, at least 50° C., preferably60° to 80° C. The amount of sodium added while adjusting the pH is mostoften too small and further sodium ions are thus requested, and arethereby added in the form of a sodium salt preferably sodium sulfateand/or sodium chloride. The sodium salt is from a dispensing point ofview preferably added in the form of an aqueous solution. The additionof Na⁺ -ions can be done either before or after the adjustment of the pHto 2.0-3.0. So added the sodium fluoroaluminate will precipitate and theprecipitate is isolated preferably by way of filtration such as vacuumand/or pressure filtration.

Sometimes, in order to increase the yield the aluminium content in theprecipitation step of cryolite is primarily held low, whereby a part ofthe fluorine content present is firstly coprecipitated as sodiumfluorosilicate. In a later part of the reaction aluminium sulphate isthen added whereby said silicate is dissolved and the fluorine set freeis reacted with the aluminium to form cryolite.

Having thus isolated the sodium fluoroaluminate (cryolite) the silicacontent of the waste products of the above described reactions, i.e.mainly silica from reaction 1 but also silica present in the motherliquor and washing water, can be recovered e.g. in the form of sodiumsilicoaluminate. Thereby a sodium silicate solution is prepared bydissolving the silica from reaction 1 in sodium hydroxide as well as analuminium sulfate solution is prepared. Said solutions are added to thefiltrate obtained after isolation of the cryolite and pH is adjusted to6 to 14, preferably 7 to 10, whereby the sodium silicoaluminateprecipitates. The sodium silicoaluminate as precipitated is isolated byfiltration e.g. vacuum and/or pressure filtration.

Under certain circumstances the fluorine content of the silica may betoo high to be used directly in the preparation of sodium aluminiumsilicate as cryolite will precipitate together therewith. Thereby it ispossible to remove the fluorine content from the silica by suspendingthe silica waste in sulphuric acid, heating the mixture, wherebyfluorine and silicofluoride are driven off, isolated in a scrubber andpassed on to the precipitation step of fluoroaluminate, and the silicais suspended in water for dissolving any aluminium sulphate present. Theslurry thus obtained is then passed to a filtration step, where thesolid, and now pure silica is separated off, washed and passed on to thedissolver for production of sodium silicate and the aluminium sulphatesolution containing sulphuric acid is passed to the precipitation stepof sodium aluminium silicate.

It is further possible to eliminate any coprecipitated phosphate in thecryolite, which coprecipitation may occur at higher pH levels in therange indicated, by slurrying the cryolite precipitated in acidicenvironment i.e. at pH 1-2.

An exemplary, preferred embodiment of the process will now be describedin more detail with reference to the accompanying drawing, the singleFIGURE of which is a flow sheet of said embodiment.

Mother liquors and washing water from the manufacture of aluminiumfluoride are optionally but preferably collected in a buffer storagetank 1 in which the pH can be adjusted to pH ˜1 and the Al:6F molarratio is adjusted to 1-1.25:1, 50% NaOH solution is stored in a storagetank 2, and a solution of Na₂ SO₄ is stored in storage tank 3. In afurther tank 4 a solution of aluminium sulphate containing 8% Al₂ O₃ isstored. The collected solution from buffer tank 1 having a temperatureof preferably 50°-70° C. to keep the formation of Na₂ SiF₆ low, is viaconduit 5 added to a reaction vessel 6 provided with heating means andstirring means (not shown). Heating is provided by a steam jacket.Sodium hydroxide solution is slowly added via a conduit 7 and theaddition is made and controlled by a pH-meter (not shown) regulating thepH of the solution added to pH 2.0-2.5 (2.25±0.25). To increase thesodium content of the reaction mixture sodium sulphate (or alternativelysodium chloride) solution is added from the tank 3 via conduit 8. Whenso adding and controlling the said additions a fine-particle solidprecipitates, which solid consists of sodium fluoro aluminate. Theaddition of sodium is so made that substantially all fluorine present inthe collected solution is eliminated.

In case the fluorine content is high and the aluminium content is low,aluminium is added to the reaction vessel 6 via a conduit 9 in the formof an aluminium sulphate solution containing about 8% Al₂ O₃, at whichconcentration the solution can be stored without risk of solid aluminiumsulphate crystallizing.

In case the collected solution has a hydrogen ion concentrationdiffering from about pH 1 the pH can be adjusted by adding sulphuricacid or sodium hydroxide to said value. By adding Al in the form ofaluminium sulphate the aluminium: 6 fluorine molar ratio can becontrolled, whereby said ratio should be 1-1.25:1. Likewise, fluosilicacid can be added to control said ratio. The pH shall be about 1 as athigher pH fluorine seems to be present as complexes which do notdissolve but remain in solution after precipitation.

The solution containing the precipitated sodium fluoroaluminate is thentransferred via a conduit 10 to a sedimentation tank and then to a vacuofiltering device 11, wherein the solid sodium fluoro aluminate isseparated off, washed, dried at about 100° C. and calcinated at 500° to700° C. and is removed to a sacking station 19 and storage for deliveryto the market. The sodium fluoroaluminate, cryolite, having a lowresidual content of phosphorus and silica is an excellent fluxing agentin the manufacture of aluminium.

In order to keep the quality of the cryolite high thereby eliminateproblems when used as a fluxing agent the residual contents of Si and Pshould be held very low. Cryolite of the present method meets thesedemands.

The filtrate obtained from the filtration apparatus can be transferredvia a conduit 12 to a second reaction vessel 13 for the production of asodium aluminium silicate.

EXAMPLE 1

A combined solution consisting of a mother liquor and washing water froma scrubber from the manufacture of aluminium fluoride comprising 15 gF/l, 3.6 g SiO₂ /l, 4.4 g Al/l, ˜50 mg P₂ O₅ /l and having a pH of ˜1was used. The Al:6F molar ratio is thereby 1.25:1.

If the Al:6F molar ratio is >1.25:1 further fluorine can be added in theform of fluosilicic acid.

To 25 liters of this combined solution Na⁺ -ions in the form of a 35%sodium chloride solution (350 g NaCl in total) are added, whereupon 370ml of 50% NaOH-solution was added slowly during 15 min to raise pH to2.2. The Na⁺ -ion excess should be 1-2 g per liter. The solution waspre-heated to 60° C. and so maintained. The reaction solution wascontinuously stirred. Sodium ions had thereby been added in at least anamount of three times the molar content of aluminium. When increasingthe pH of the solution a precipitate consisting ofnatriumfluoroaluminate was obtained. The reaction time to complete theprecipitation is 1 to 3 hr. The solution containing the precipitatebeing dispersed therein was transferred to a vacuum filtrationapparatus, where the sodium fluoroaluminate was separated off, washedwith water, dried at about 100° C., calcinated at 500° to 700° C. andpacked.

Yield: Sodium fluoroaluminate: 650 g containing 55.6% F, 28.8% Na, 14.8%Al, and 0.4% SiO₂, and 0.03% P₂ O₅.

EXAMPLE 2

A combined solution inance with that of Example 1 was treated in thesame way as the solution was treated in Example 1 above with thefollowing exception. When the reaction had went on for about 20 minutes13.8 g of aluminium was added in the form of an aluminium sulphatesolution, whereafter the reaction was continued for another 1.5 hr.

Residue SiO₂ in the sodium fluoroaluminate was thereby reduced to 0.07%.All other analytical data corresponded to those obtained in Example 1.

EXAMPLE 3

2 liters of a combined solution comprising 18.5 g F/l, 10 g Si₀ 2/l, 5.0g Al/l, ˜50 mg P₂ O₅ /l and having a pH of ˜1 were used. The Al:6F molarratio was thereby 1.15.

The solution was treated in accordance with Example 1, step 1, aboveusing 28 g NaCl, and 30 ml of 50% NaOH-solution. The end pH was 2.3, thereaction temperature 60° C., and the reaction time 3 hr.

Yield of sodium fluoroaluminate 64 g containing: 30.8% Na, 52.1% F,12.8% Al, and 0.58% SiO₂ and 0.04% P₂ O₅ (100% cryolite).

The sodium fluoroaluminate prepared can be obtained in the form ofcryolite alone, chiolite alone, or mixtures thereof, depending on theorder in which the reactants are added together. Having the wholecontents of sodium ions present from the beginning provides for aproduction of cryolite, whereby a late addition of sodium ions providesfor a formation of chiolite. This will be evident from the followingExamples 4 to 9.

EXAMPLE 4

4 l of a combined mother liquor and scrubber water solution of thecomposition given in Example 1 were added to a reaction vessel,whereupon 35 g NaCl were added and the total mixture was brought to atemperature of 60° C. Thereafter sodium carbonate was added to pH 2.5.The precipitate obtained consisted of sodium fluoroaluminate in the formof 100% cryolite.

EXAMPLE 5

4 l of a combined solution in accordance with Example 4 above andcontaining 35 g of NaCl were added simultaneously with a sodiumcarbonate aqueous solution (58.5 g of Na₂ CO₃) to a reaction vessel. Thetemperature of the solutions was kept at 60° C. Final pH was 2.5. Theprecipitate obtained consisted of sodium fluoroaluminate in the form of75% cryolite, and 25% chiolite.

EXAMPLE 6

4 l of a combined solution in accordance with Example 4 above wereneutralized to pH 2.5 using sodium carbonate, whereupon 35 g of NaClwere added. The temperature of the solutions was 60° C. The precipitateobtained consisted of sodium fluoroaluminate in the form of 100%chiolite.

EXAMPLE 7

4 l of a combined solution in accordance with Example 4 above wereheated to 60° C. and placed in a reaction vessel. To this solution 35 gof NaCl and sodium carbonate to pH 2.5 were added simultaneously. Theprecipitate obtained consisted of sodium fluoroaluminate in the form of100% chiolite.

EXAMPLE 8

To a reaction vessel an aqueous solution of sodium chloride (35 g) wasadded, whereupon sodium carbonate and 4 l of a combined solution inaccordance with Example 4 having an Al:6F molar ratio of 1.05:1 wereadded simultaneously. The precipitate obtained consisted of sodiumfluoroaluminate in the form of 100% cryolite.

EXAMPLE 9

Cryolite was prepared in accordance with Example 1, whereafter, whenstill in the form of a slurry, sodium carbonate and H₂ SiF₆ were addedsimultaneously to obtain an Al:6F molar ratio of 1.05 while maintainingpH of 2.5. Yield 100% of cryolite.

In case chiolite has been obtained the NaF:AlF₆ weight ratio can becorrected to fulfil the requirements set forth by the aluminium industryusing cryolite as a fluxing agent whereby chiolite or chiolite/cryoliteis granulated together with sodium carbonate to give a correct weightratio, viz. 1.5.

Sodium fluoroaluminate can be prepared in a continuous process accordingto the present invention as well, whereby such a process will beexemplified below.

EXAMPLE 10

The combined solution identified (Ex 4) is stored in a buffer tank at60° C. from where it is pumped to a first reaction vessel, from there toa second reaction vessel, from there to a third reaction vessel fromwhich any precipitate formed is drawn off and allowed to settle in afourth reaction vessel. The addition of the reactants can be made to thedifferent reaction vessels in different orders depending on the finalcompound/product wanted.

The combined solution used in a series of tests of the continuousproduction contained as follows: F 12.0 g/l; Al 3.6 g/l; SiO₂ 2.57 g/l;Ca 210 mg/l; P₂ O₅ 60 mg/l.

Al:6F molar ratio 1.26.

Sodium chloride solution: 200 g NaCl/l.

Technical grade of sodium carbonate.

    __________________________________________________________________________    Test parameters                                                                                 Addition                                                                           Addition                                                  Addition                                                                           Addition                                                                           Addition                                                                           of NaCl                                                                            of                                                        of comb.                                                                           of NaCl                                                                            of NaCl                                                                            to comb.                                                                           Na.sub.2 C0.sub.3                                                                  Reaction                                          Test                                                                             solution                                                                           solution                                                                           to vessel                                                                          solution                                                                           to vessel                                                                          temp Cryolite                                                                           Chiolite                                no ml/min                                                                             ml/min                                                                             no   g/l  no   °C.                                                                         %    %                                       __________________________________________________________________________    1  170  16.7 1    20   1    50   93    7                                      2  170  16.4 2    20   1    50   40   60                                      3  173  16.3 1    20   1    60   93    7                                      4  173  33.2 1    40   1    50   100   0                                      5  170  --   Buffer                                                                             12   1,2,3                                                                              50   10   90                                                   tank                                                             6  170  15   1      18.8                                                                             2    60   17   83                                      __________________________________________________________________________    Analyses of final products                                                    Test  H.sub.2 O                                                                            Na    Al    F     SiO.sub.2                                                                           P.sub.2 O.sub.5                          no    350° C.                                                                       %     %     %     %     %                                        __________________________________________________________________________    1     3.11   31.1  12.6  52.4  0.69  0.12                                     2     2.57   28.9  13.3  54.2  0.79  0.11                                     3     1.95   30.5  12.3  53.2  0.21  0.15                                     4     2.24   30.7  12.5  52.8  0.46  0.12                                     5     3.00   26.5  14.5  53.5  --    --                                       6     3.06   26.7  14.4  53.2  --    --                                       __________________________________________________________________________    Analyses of filtrate                                                          Test  Na     F     Al    SiO.sub.2                                                                           P.sub.2 O.sub.5                                no    g/l    g/l   g/l   g/l   mg/l  pH                                       __________________________________________________________________________    1     6.3     1.4   1.0   2.4  33    2.45                                     2     5.8     1.4   0.95  2.3  33    2.48                                     3     5.6     1.8   1.1   2.5  36    2.46                                     .sup. 4.sup.x                                                                       12.1    0.12  0.05  3.4  25    2.50                                     5     5.0     1.6   0.6   2.3  35    2.43                                     6     4.8     1.7   0.58  2.3  38    2.11                                     __________________________________________________________________________     .sup.x Cryolite suspension adjusted with hexafluorosilicic acid to Al/6 F     = 1.05 and sodium carbonate to pH 2, the contents prior to the adjustment     being Na 12.1%; F 1.5%; Al 0.95; SiO.sub.2 2.2; pH 2.52.                 

I claim:
 1. A method for recovering usable products derived from themanufacture of aluminum fluoride from H₂ SiF₆ and Al(OH)₃ to form liquid--AlF₃, and thereafter treating said --AlF₃ to form insoluble, anhydrousAlF₃, said manufacture leaving mother liquors and washing waterscontaining aluminum, and fluorine in amounts below the solubility limitof anhydrous AlF₃ and containing contaminating silica, characterized bycollecting the mother liquors and washing waters, and adding alkalimetal ions in the form of an alkali metal salt, adjusting the hydrogenion concentration of said collected solution to a pH of 2.0 to 3.0 byadding an alkali metal base while maintaining a temperature of saidcollected solution above about 50° C., and isolating the alkali fluoridealuminate thereby precipitated.
 2. A method according to claim 1,characterized in that sodium hydroxide and a sodium salt, are used informing a sodium fluoroaluminate.
 3. A method according to claim 2,wherein the sodium salt is selected from the group consisting of sodiumsulfate and sodium chloride.
 4. A method according to claim 1,characterized in that the pH is 2.0-2.5.
 5. A method according to claim1, characterized in that the molar ratio of Al:6F in the collectedsolution is 1-1.25:1.
 6. A method according to claim 1, characterized inthat the aluminium content of the reaction mixture is adjusted afteraddition of the base and the alkali metal salt.
 7. A method according toclaim 6, characterized in that the aluminium content is increased aftera first precipitation of fluoroaluminate, to precipitate furtherfluoroaluminate from fluorine set free at said increase.
 8. A methodaccording to claim 1, characterized in that pH of the collected solutionis controlled to be about pH 1 prior to adjusting it to pH 2.0-3.0.
 9. Amethod according to claim 1, characterized in that the temperature isheld between 50° and 90° C.
 10. A method according to claim 9, whereinthe temperature is held between 60° to 80° C.
 11. A method according toclaim 1, which further comprises treating the resultant filtratecomprising mainly silica, and minor amounts of aluminium and fluorine torecover one or more of said compounds.